In a multipath environment, multipath echoes originating from the same transmission signal arrive at a receiving point through different paths due to reflection from a tall building or diffraction by the straight edge or the corner of an obstruction. Accordingly, a received signal contains a direct wave propagating through a straight (the shortest) path, followed by delayed waves arriving slightly after the direct wave. Radio communication that can expect a direct wave is called line-of-sight (LOS) communication because of the straight path between transmission and receiving antennas along a visual line of sight. In contrast, radio communication that cannot expect a direct wave is called Non-line-of-sight (NLOS) communication, in which a transmission antenna cannot see the receiving antenna due to LOS blocks. In NLOS communication, the first arriving wave is also an NLOS delay signal subjected to reflection or diffraction.
Locating positions using radio signals that are susceptible to multipath propagation has been an important issue for wideband mobile communication systems. The conventional approach for performing positioning is based on the delays of first arriving signals only, which approach is to reduce the multipath positioning problem to the conventional single-path model. One of known positioning techniques for locating the position of a mobile station based on first-arrival detection is trilateration (electro-optical distance measuring) that makes use of radio signals transmitted between a base station and the mobile station.
On the other hand, a technique for creating a multipath delay profile from a received signal to improve the positioning accuracy using the delay profile is proposed (by, for example, JP 2001-298763A and JP 2003-194916A). In JP 2001-298763A, delay variance is estimated from the created delay profile, and the positioning accuracy is determined based on the variance. In JP 2003-194916A, interference signals are cancelled using a delay profile, prior to performing trilateration-based position estimation, to improve the positioning accuracy.
Furthermore, for underwater application, an acoustic positioning scheme adapting the full multipath structure of received signals has been proposed. See M. Deffenbaugh, H. Schmidt and J. G. Bellingham, “Acoustic Positioning in a Fading Multipath Environment,” Proc. MTS/IEEE ‘Prospects for the 21st Century’ (MTS/IEEE Oceans 1996), Vol. 2, pp. 596-600, September 1996. The method disclosed in this publication consists of two steps. First, two sets of multipath delay estimates, delay estimates using actual received signals and estimates predicted by a so-called ray tracing model, are produced. Then, the differences between the measured and predicted estimates are used in a linear inversion to obtain a position estimate.
Meanwhile, a signal processing technique is proposed for improving the accuracy performance of a position-fixing navigation system when the geometry of the navigation landmarks (e.g., sensors) is nearly collinear. See R. J. Kelly, “Reducing Geometric Dilution of Precision Using Ridge Regression,” IEEE Transactions on Aerospace and Electronic Systems, Vol. 26, No. 1, January 1990. This techniques is irrelevant to use of multipath components contained in a received signal, but discloses a method of modifying the conventional least mean square (LMS) position-fixing algorithm using ridge regression, which method is applied to signal processing when landmarks (transmitters) are in a particular geometry.
The conventional techniques disclosed in the above-described publications have some problems. In JP 2001-298763A and JP 2003-194916A, the delay profile is used only for the purpose of determining the accuracy or reducing interference, and the delay components themselves are not reflected in position estimation, but carry out position estimation based only on the first arriving wave. The ray tracing model used in the underwater application with an acoustic positioning scheme is difficult to apply to mobile communication channels, and cannot achieve optimization.